Electronic devices are becoming increasingly smaller and faster, as a result of which high-density packaging and high-density wiring of a multilayer printed wiring board are promoted. This requires that wirings provided in the multilayer printed wiring board are thinly formed and through-holes provided therein are minutely formed. In order to satisfy these requirements, a buildup-type multilayer printed wiring board is used.
A rigid double-sided substrate or multilayer substrate in which a fiber base member is utilized is widely used as an internal layer which serves as a core of the multilayer printed wiring board. Further, insulating resin layers to be laminated on such a substrate are provided using a method of repeatedly coating a resin material onto the substrate, a method of laminating resin films onto the substrate or a method of laminating copper foils with the resin material onto the substrate.
In this case, in general, the fiber base member is not embedded in each of the insulating resin layers laminated. However, it has become clear that the buildup-type multilayer printed wiring board having such a structure has the following problems. Namely, cracks easily occur within the resin layer (hereinafter, referred to as “buildup layer” on occasion) in which the fiber base member is not embedded. Further, in the case where thermal shock is applied to the buildup layer, cracks also easily occur therewithin due to thermal expansion and contraction of the resin material constituting the buildup layer.
For these reasons, it has been examined that the fiber base member is embedded in the buildup layer. However, in the case where the buildup layer in which the fiber base member is embedded is used, when through-holes (via-holes) are formed therethrough using laser, smear is generated on a surface of the buildup layer or inner surfaces of the through-holes, as a result of which circuit deficits occur. Hereinabove, in order to improve laser workability of the buildup layer in which the fiber base member is embedded, a method of using an organic fiber base member as the fiber base member is proposed (see, for example, Japanese Patent Application Laid-open No. Hei 11-330707).
However, in the case of the method of using the organic fiber base member, since the organic fiber base member has only low rigidity, it is sometimes difficult to make the buildup-type multilayer printed wiring board thinner.